Nokia just posted its quarterly results - including shipped devices - and it's not looking good. Massive losses, sales dropping, and no growth in Lumia sales in the US. The company is losing money hand-over-fist, and with Windows Phone 8 still months away, the company warns the next quarter will be just as bad.

Installed base is most relevant to developers, and developers can target both WP7 and WP8 by just making a WP7 app. So the installed base won't start from zero.

Developers could choose to develop for the old XNA/Silverlight API, or the new WinRT API. Question is, what makes more sense. The Advantage of XNA/Silverlight is that your app will work on WP7.
If you develop for WinRT however, that means you get (among other things) proper multitasking, native code support, and apps that work on the Windows RT tablets. And you will develop using the API that is en vogue at Microsoft, the importance of which should not be underestimated.

Developers targeting WP8 exclusively in the beginning will probably be large game studios (because of DirectX and high end hw support), but for them WP7 was suboptimal anyway.

I expect that almost nobody will target WP8 exclusively. It's either WP7+WP8 or WP8+Windows RT.

Silverlight is already legacy and will not be supported in the future. It would be silly to build up anything new on it. The success of Silverlight is, well, limited. There are just very less things using Silverlight. The Silverlight and Flash kind of browser plugin turned out to have no future with HTM5. WinRT at least has Microsofts support and works fine on all current Microsoft platforms.